Clathrate hydrates are crystalline compounds that occur when water forms a cage-like structure around guest molecules, particularly gaseous molecules. Clathrate hydrates, especially in the petroleum industry, are referred to as gas hydrates, gas hydrate crystals, or simply hydrates. For purposes of this application, these terms will be used interchangeably. In the petroleum industry, gas hydrates pose particular problems with respect to producing, transporting, and processing of hydrocarbons. Typical gas hydrates formed in petroleum (hydrocarbon) environments are composed of water and one or more guest molecules such as methane, ethane, propane, isobutane, normal butane, nitrogen, carbon dioxide, and hydrogen sulfite. It is also known that other guest molecules such as ethylene, nitrous oxide, acetylene, vinyl chloride, ethyl bromide, and oxygen can form clathrate hydrates.
When allowed to form and grow, gas hydrate crystals can become a nuisance at best and pose a serious problem at worst. Gas hydrates can block transmission lines and plug blowout preventers, jeopardize the foundations of deep water platforms and pipelines, collapse tubing and casing, and taint process heat exchangers and expanders. To overcome these problems, several thermodynamic measures are possible in principal: removing free water, maintaining an elevated temperature and/or reduced pressure, or adding freezing point depressants (antifreeze). As a practical matter, adding freezing point depressants has been most frequently applied. Lower alcohols and glycols such as methanol have been added to act as antifreezes. However, in order for such substances to be effective, it is necessary that they be added in substantial amounts, such as 30% by weight of the water present. Not only is this expensive, but it also poses an additional problem since the addition of such large amounts of antifreeze requires recovery prior to further processing of the fluid mixture.
It has been known for some time that in lieu of antifreeze, one can employ a crystal growth inhibitor that inhibits the formation of gas-hydrate crystals and/or the agglomeration of gas hydrate crystallites to large crystalline masses sufficient to cause plugging. Surface active agents such as phosphonates, phosphate esters, phosphonic acids, salts and esters of phosphonic acids, inorganic polyphosphates, salts and esters of inorganic polyphosphates, polyacrylamides, and polyacrylates have been used. It is also known that poly-N-vinyl-2-pyrrolidone (PVP), which is a well-known water-soluble polymer, is effective, in relatively low concentrations, in interfering with the growth of gas-hydrate crystals. W094/12761, published Jun. 9, 1994, discloses other additives for inhibiting crystal growth and controlling the formation of gas hydrates in fluid mixtures such as those encountered in the hydrocarbon industry in the production, transportation, and processing of petroleum and natural gas fluids. It has also been disclosed that certain polyglycol diamines prevent blockage of gas transmission lines with gas hydrates. These amines have the following general structure: H.sub.2 NCHRCH.sub.2 (OCH.sub.2 CHR).sub.n NH.sub.2 where R=H or CH.sub.3 and n=2 to 6.
However, compositions and methods known in the art are only partially effective in preventing the formation of gas hydrates. When applied to conditions that strongly favor hydrate formation, prior art compositions and methods lose their effectiveness, to the point that they are not a practical solution to the hydrate problem.
The compositions and methods of the present invention are advantageous and more desirable than those of the prior art in that they are more effective at lower concentrations and at temperatures several degrees below the hydrate formation point. Moreover, they display a synergistic effect when co-applied with kinetic, polymer type, hydrate inhibitors.